1. Field of the Invention
The present invention relates to formation of an intermetallic layer on a metal component and, more particularly, to formation of an intermetallic layer on the airflow surface of a jet engine metal component.
2. Description of Prior Art
The surface of metal components is often desirably treated to form an intermetallic layer thereat by which to protect the underlying metal component and thereby prolong its useful life. By way of example, in the aerospace industry, many of the components in a jet engine or other aspect of a plane are provided with an aluminide layer to protect the airflow surfaces from corrosion. Over time, the aluminide layer will wear and need to be repaired. In those cases, any oxide layer and remaining aluminide or other intermetallic layer on the component is removed such as by stripping in acid and/or gritblasting to reveal an underlying surface of the metal component. The metal component, such as a nickel-based or cobalt-based superalloy jet engine component, is then placed in a simple CVD furnace, for example, and exposed to a deposition environment such as near vacuum and high heat with appropriate activators and donor materials from which to form the intermetallic layer. Where the intermetallic layer is to be an aluminide, the donor material may be aluminum in the form of chromium-aluminum or cobalt-aluminum chunklets, for example. In the deposition environment, the aluminum frees from the chunklets and forms a nickel-aluminide layer on the nickel-based superalloy component (which layer may be referred to simply as an aluminide layer, for shorthand). The aluminide layer includes an additive portion growing outwardly of the original metal surface of the component and which has a high concentration of aluminum. The aluminide layer may also include a diffusion portion extending partially into the component inwardly of the level of the original surface and which will have a high concentration of the component metal, such as nickel. This same process may be used for new components after removal of the natural oxide layer which might form on the component when it is first manufactured.
The intermetallic layer is to be formed or grown to a desired overall thickness by exposing the component, and especially its surface, to the deposition environment for a predetermined time sufficient to form the layer. The length of time necessary to run the simple CVD furnace through a complete cycle necessarily limits the number of parts that can be processed through that furnace in a given period of time, such as a workshift. Shortening the cycle time would be advantageous in that more parts could be processed over a workshift, for example, thereby reducing costs on a per part basis. Unfortunately, while the process variables may be adjusted in ways which might slightly affect the time required to form the desired thickness of the intermetallic layer, efforts to substantially reduce the time typically require undesired process variable changes. Those process variable changes can prove undesirable from a cost or safety standpoint and/or from a product standpoint. Thus, there remains a need to reduce cycle time but without undesirable changes to the process variables involved in the deposition environment.
In addition to the above, there are some situations where it is desirable to form a multi-component intermetallic layer, i.e., an intermetallic layer that includes a functional material other than just from the donor (e.g., aluminum) or the component (e.g., nickel). In the aerospace industry, for example, it has long been desired to include silicon, chromium or platinum in the aluminide layer, so as to enhance the performance characteristics of the intermetallic coating layer. Current efforts to include silicon are largely unacceptable. And while addition of chromium or platinum has been accomplished, the process involved in the addition of those materials has been complex and costly. By way of example, platinum may be added by first electroplating the clean metal surface with platinum prior to exposing the part to the deposition environment for the formation of the aluminide layer. It is thought that during the deposition of the aluminide layer, the platinum atoms free from the plating and migrate into the aluminide layer thereby providing a desirably strong and durable platinum aluminide deposition layer. While the addition of the platinum provides a desirably improved metal component in terms of its durability and useful life, electroplating a product with platinum is an expensive and difficult procedure. Hence, there remains the need to easily and inexpensively add an additional functional material to the intermetallic layer to form a multi-component layer.
The present invention provides an improved deposition process by which to form an intermetallic layer on a metal component which overcomes some of the above-noted drawbacks. To this end, and in accordance with the principles of the present invention, an inoculant is first applied to the surface of the metal component at which the intermetallic layer is to be formed. The inoculant may be applied to the entire surface or may be applied selectively to one or more surface portions of the metal component. The inoculant is advantageously applied in a liquid state and then dried to form a pre-coat of the inoculant. The pre-coated component is then placed into the deposition environment where the intermetallic layer is formed. It is found that the intermetallic layer grows or forms more quickly at the pre-coated surface, than would have occurred without the inoculant. Thus, a thicker intermetallic layer forms in an area of the component that was pre-coated with the inoculant as compared to an area that was not pre-coated. As a result, the desired thickness of the intermetallic layer may be formed in a reduced period of time as compared to a conventional deposition process. That result may be used to advantageously reduce the cycle time of the simple CVD furnace which provides the desired benefits in cost savings and the like. Alternatively, a thicker intermetallic layer may advantageously be formed where the cycle time is not substantially reduced with a pre-coated component as compared to a component that was not pre-coated. It will thus be appreciated that as used herein, the term inoculant refers to a material that when applied to a metal surface which is then exposed to a deposition environment, will cause an intermetallic layer to form at the surface more quickly or to a greater thickness than would occur without the inoculant. Advantageously, the inoculant may be a silane material or a metal-halogen Lewis acid material, by way of example,
In addition to the foregoing, it is possible to form two different thicknesses of intermetallic layer on the same component, depending upon which portion thereof is pre-coated with the inoculant. By selectively coating the component, a desirably thick intermetallic layer may be formed on the areas of the component which need the most protection, while providing a thinner layer on areas less susceptible to damage such as from corrosion. In a particular application, the inoculant may be applied to the air flow surface(s) of a jet engine component (such as a blade) to subsequently form a desirably thick aluminide coating in these areas. Other portions of the blade, such as those which might abut other components in the engine are not pre-coated and so will result in a thinner intermetallic layer in those areas.
In accordance with a further aspect of the present invention, applying a liquid inoculant coating may be done simply by dipping the part or by spraying or brushing the liquid inoculant onto the part, either completely or selectively, which thus allows for application of coating not only to the exposed, readily viewable surfaces, but also to the internal surfaces, such as a hollow interior of a cooling hole or passage in a jet engine blade. As a consequence, the inoculant can be provided on internal surfaces otherwise not readily plated to thereby enhance the growth of the intermetallic layer thereat to thus protect those surfaces and prolong the useful life of the metal component.
In accordance with a yet further aspect of the present invention, the inoculant may be used to easily and inexpensively add additional functional material to the intermetallic layer to thus provide the sought-after multi-component layer. Thus, where the inoculant is a silane material, silicon is advantageously diffused into the intermetallic layer during formation in the deposition environment. Similarly, where the innoculant is a metal-halogen Lewis acid, the metal ion of the Lewis acid may be selected for its beneficial properties in connection with the intermetallic layer. Thus, for example, the Lewis acid may be CrCl3, PtCl4, ZrCl4, or ZrF4 to thus include the metal ions of either chromium, platinum, and/or zirconium as the additional functional material in the intermetallic layer. When the part with such a Lewis acid inoculant thereon is exposed to the deposition environment, it is believed that the halogen (i.e., the chlorine or flourine) becomes part of the reactant gas, and the chromium, platinum and/or zirconium ions, for example, will free from the inoculant and migrate into the intermetallic layer, such as an aluminide layer, being formed on the metal component to thereby produce a desired chromium aluminide, platinum aluminide, and/or zirconium aluminide layer with its advantageous properties. However, the Lewis acid inoculant is applied more easily and thus less expensively than a platinum or chromium plating, and is also a much lower cost material than is platinum or chromium used for plating.
Where the inoculant is a Lewis acid of the metal-halogen type, there may be some metal components which will experience grain boundary problems at the surface in the deposition environment. In accordance with a further aspect of the present invention, the advantage of the Lewis acid inoculant may be obtained without such grain boundary problems by application of a fine powder of the desired donor metal to the Lewis acid on the component while still in the liquid state. By way of example, aluminum powder may be sprayed onto the liquid Lewis acid on the surface. When the component with the Lewis acid inoculant and added donor metal is in the deposition environment, the grain boundary problem is reduced or minimized.
In accordance with a still further aspect of the present invention, the inoculator may be selectively applied to aerospace components and particularly jet engine components such as blades, shrouds, and vanes to name a few. Such components have portions exposed to the high-pressure air flow path of the engine where an intermetallic layer, and a possibly multi-component intermetallic layer, is desired. At the same time, other portions of those aerospace components are not in the air flow path and so do not need the same level of protection in use. In some situations, the growth of more than a thin intermetallic layer can be detrimental, particularly with respect to those portions of the component that contact other engine components and must thus fit together in close tolerances. In such situations, the inoculant may be selectively applied to those portions of the component adapted to be exposed to the high-pressure air flow, so as to permit growth of the desirable thick and/or multi-component intermetallic layer on those portions. The remaining portions of the component may either be shielded as conventional, or permitted to grow an intermetallic layer which will, however, be thinner than that formed in the pre-coated areas due to the lack of the pre-coating of inoculant thereon.
By virtue of the foregoing, there is thus provided an improved deposition process by which to form an intermetallic layer on metal components. These and other objects and advantages of the present invention shall become apparent from the accompanying drawings, and the description thereof.